1. Technical Field
The present disclosure relates to a package for an optical semiconductor element and more particularly to a package for an optical semiconductor element having an excellent property in high-speed communication.
2. Related Art
In a communication field, a value of characteristic impedance of a signal wire is unified as 50Ω. As for a package for an optical semiconductor element used for optical communication, it was not necessary to pay attention to matching of the characteristic impedance in the past. However, in high-speed communication at 8 Gbps, for example, a problem occurs in that signal transmission efficiency deteriorates if the value of the characteristic impedance deviates from 50Ω. Therefore, the characteristic impedance is required to be matched.
A signal wire (lead pin) used for the package for the optical semiconductor element is attached to a stem body by glass-sealing and the signal wire becomes a pseudo-coaxial structure. The present invention relates to a technique capable of matching the characteristic impedance in a portion where the lead pin is sealed.
Characteristic impedance Zo of a signal lead pin is given by the following expression:Zo=(138/Er1/2)×log(D/d),where Er is a dielectric constant of glass used for sealing the lead pin, D is a hole diameter of a sealing hole for mounting the lead pin, and d is a wire diameter of the lead pint.
The shape, size, and the like of the packet for the optical semiconductor element are various. In a general package for an optical semiconductor element (for example, TO-46 type) used for optical communication, the dielectric constant Er is 5.0, the sealing hole of the lead pin is 1.2 mm, the wire diameter of the lead pin is 0.45 mm, and the value of the characteristic impedance is 25Ω. Accordingly, in the package for the optical semiconductor element in which the value of the characteristic impedance deviates from 50Ω, it is necessary to correct the value of the characteristic impedance to 50Ω in order to improve a signal transmission property (see e.g., JP-A-2007-220843).
Also, as a method of enabling the value of the characteristic impedance of the package for the optical semiconductor element to be close to 50Ω, a method of increasing the hole diameter D of the sealing hole of the lead pin, a method of decreasing the wire diameter of the lead pin, and a method of decreasing the dielectric constant Er of sealing glass are taken into consideration.
Among these methods, the method of increasing the hole diameter D of the sealing hole of the lead pin results in reduction in an area where an optical semiconductor element of the step body is arranged due to the increase in the sealing hole. Therefore, the size of the hole diameter D is restrictive due to the specification of the stem body.
In addition, in order to realize the method of decreasing the wire diameter of the lead pin, the wire diameter has to be set in consideration of a wire bonding property, since an end surface of the lead pin is a wire bonding surface.
The method of using glass having a low dielectric constant Er as sealing glass also has a problem in that glass having the low dielectric constant is generally poor in a sealing property and an airtight property is problematic when the lead pin is sealed. In general, when the dielectric constant of glass is decreased, modifier oxide (metal oxide or the like) added to the glass has to be reduced. For that reason, an amount of an additive contributing to the airtight property with the lead pin or the stem body is reduced, and thus the airtight property with the lead pin or the like deteriorates. Moreover, since the additive has a function of adjusting a thermal expansion coefficient of glass, the airtight property may deteriorate in a package (matched type) for an optical semiconductor element in which the thermal expansion coefficient of the glass varies by changing the amount of additive and the thermal expansion coefficient of the glass is matched with the stem body.